Semiconductor light emitting device

ABSTRACT

A semiconductor light emitting device of double hetero junction includes an active layer and clad layers. The clad layers include an n-type layer and p-type layer. The clad layers sandwich the active layer. A band gap energy of the clad layers is larger than that of the active layer. The band gap energy of the n-type clad layer is smaller than of the p-type clad layer.

This application is a divisional application of Ser. No. 08/528,308filed Sep. 14, 1995 and now U.S. Pat. No. 5,751,752.

BACKGROUND OF THE INVENTION

The invention relates to a semiconductor laser having a doubleheterodyne structure. More particularly, the invention relates to asemiconductor laser which uses a semiconductor of gallium nitride typecompound suitable for emission of blue light, which is capable ofreducing operating voltage without reducing the light emittingefficiency.

In the past, blue LED had a fault in putting it to practical use becauseit has lower luminance than a red LED or a green LED, but in recentyears the luminance of the blue LED has increased and is in thespotlight now as a semiconductor of gallium nitride type compound hasbeen in use, making it possible to obtain in p-type semiconductor layerof a low resistance containing Mg as a dopant.

The semiconductor of gallium nitride type compound described here isreferred to a semiconductor in which a compound of Ga of group IIIelement and N of group V element or part of Ga of group III element issubstituted by other group III element such as Al and In and/or asemiconductor in which part of N of group V element is substituted byother group V element such as P and As.

In a conventional manufacturing method, gallium nitride type LEDs weremanufactured in such processes as described below, and a perspectiveview of LED which uses a semiconductor of completed gallium nitride typecompound is shown in FIG. 11.

First, by the organometallic compound vapor phase growth method(hereinafter referred to as MOCVD method), the carrier gas H₂ togetherwith trimethyl gallium which is an organometallic compound gas(hereinafter referred to as TMG), ammonia (NH₃) and SiH₄ and the likeare supplied as a dopant to a substrate consisting, for example, ofsapphire (single crystal Al₂ O₃) at low temperature of 400° to 700° C.,approximately 0.10 to 0.2 μm of low temperature buffer layer 2consisting of n-type GaN layer is formed, and then the same gas issupplied at high temperature of 700° to 1200° C., and approximately 2 to5 μm of high temperature buffer layer 3 consisting of n-type GaN of thesame composition is formed. The low temperature buffer layer 2 is formedby polycrystalline layer to ease the strain caused by mismatching of thelattice between a substrate 1 and the single crystal layer ofsemiconductor and then turned into a single crystal by being subjectedto 700° to 1200° C., in order to match the lattice by laminating thesingle crystal of the high temperature buffer layer 3 on that singlecrystal.

Further, material gas of trimethyl aluminium (hereinafter referred to asTMA) is added to the foregoing gas, a film of an n-type Al_(k) Ga_(1-k)N (0<k<1) layer containing S of n-type dopant is formed, so thatapproximately 0.1 to 0.3 μm of an n-type clad layer 4 is formed to forma double heterodyne junction

Then, instead of TMA which is the foregoing material gas, trimethylindium (hereinafter referred to as TMI) is introduced to formapproximately 0.05 to 0.1 μm of an active layer 5 consisting, forexample, of In_(y) Ga_(1-y) N (0<y<1), a material whose band gap issmaller than that of the clad layer.

Further, impurity material gas is substituted by SiH₄ using the samematerial gas used for forming the n-type clad layer 4, Mg as a p-typeimpurity of biscyclopentadiene magnesium (hereinafter referred to as Cp₂Mg) or dimethyl zinc (hereinafter referred to as DMZn) for Al is addedand introduced into a reaction tube, causing a p-type Al_(k) Ga_(1-k) Nlayer which is a p-type clad layer 6 to be grown in vapor phase. By thisprocess, a double hetero junction is formed by the n-type clad layer 4,active layer 5, and p-type clad layer 6.

Next, in order to form a contact layer (cap layer) 7, Cp₂ Mg or DMZn issupplied as the impurity material gas using the same gas as theforegoing buffer layer 23 to form 0.3 to 2 μm of the p-type GaN layer.

Afterward, a protective film such as SiO₂ and Si₃ N₄ is provided allover the surface of the grown layer of a semiconductor layer, aniline orelectron is irradiated at 400° to 800° C. for approximately 20 to 60minutes to activate the p-type clad layer 6 and the contact layer (caplayer) 7, after the protective film is removed, resist is applied andpatterning is provided to form an electrode on the n-side, part orrespective grown semiconductor layers is removed by dry etching so as toexpose the buffer layer 3 or the n-type clad layer 4 which is the n-typeGaN layer, an electrode 8 on the n-side and an electrode 9 on the p-sideare formed by sputtering and the like, and AN LED chip is formed bydicing.

As a conventional semiconductor laser, one that uses a semiconductor ofGaAs type compound is known, in which a resonator is formed by a doublehetero junction structure with both sides of an active layer being heldbetween clad layers consisting of a material having greater band gapenergy and smaller refractive index than the material of such activelayer, so that it is possible to obtain the light oscillated in suchresonator. Shown in FIG. 12 is an example of a semiconductor laser whichuses a semiconductor of GaAs type compound having a refractive indexwave guide structure provided with a difference of refractive index byan absorption layer in order to confine the light in the stripe portionof the active layer.

In FIG. 12, the numeral 121 represents a semiconductor substrateconsisting, for example, of an n-type GaAs, on which are laminated inorder a lower clad layer 124 consisting, for example, of an n-typeAl.sub.α Ga₁₋α As (0.35≦α≦0.75), an active layer 125 consisting, forexample, of Al.sub.β Ga₁₋β As (0<β≦0.3) of non-doping type or an n-typeor a p-type, a first upper clad layer 126a consisting of a p-typeAl.sub.α Ga₁₋α As, a current laminating 120 consisting of an n-typeGaAs, a second upper clad layer 126b consisting of a p-type Al.sub.αGa₁₋α As, and a contact layer (cap layer) 127 consisting of a p-typeGaAs, and the p-side electrode 128 and the n-side electrode 129 arerespectively provided on the upper surface and the lower surface inorder to form a chip of a semiconductor laser. In this structure, thecurrent limiting layer 120 consisting of the n-type GaAs restricts theinjection current to the stripe-like active area of width W, byabsorbing the light generated by the active layer, a difference ofrefractive index is provided in the inside and the outside of thestripe. Therefore, the semiconductor laser of the present invention isused as a semiconductor laser of a red or infrared ray refractive indexwave guide structure wherein the light is confined in transversedirection and the wave of stripe-like active area of width W is directedstably.

In the semiconductor laser of such structure, a blue light radiatingsemiconductor laser using a semiconductor of gallium nitride typecompound is also requested.

In a conventional semiconductor of gallium nitride type compound, thelight emitting efficiency of the light emitting element of double heterojunction is high but the operating voltage thereof is high. If amaterial of small band gap energy, that is, a material of small Alcomposition rate k of Al_(k) Ga_(1-k) N is used for the n-type cladlayer and the p-type clad layer in order to lower the operating voltage,the operating voltage is lowered but the electron outflow from theactive layer to the p-type clad layer increases, while the lightemitting efficiency is lowered.

In the case where a semiconductor laser is to be composed by using asemiconductor of gallium nitride type compound, by providing a structurewherein an active layer is interposed between both sides by a clad layerconsisting of a material having greater band gap energy and smallerrefractive index than such active layer so as to confine the light inthe active layer for oscillation, it can be considered to use In_(y)Ga_(1-y) N (1<y<1, where y=0.1 for example) as the active layer andAl_(k) Ga_(1-k) N (0<k<1, where k=0.2 for example) as the clad layer ofboth sides.

In a semiconductor laser which uses a conventional semiconductor ofarsenic gallium type compound, specific resistance of Al.sub.α Ga₁₋α Asas the clad layer is approximately 100Ω·cm and there occurs no problemof increased operating voltage or heat generation even if such cladlayer is used as one requiring approximately 1 to 2 μm, but if asemiconductor of gallium nitride type compound is used, the specificresistance of Al_(k) Ga_(1-k) N (k=0.2 for example) is approximately1000Ω·cm when the carrier density of 10¹⁷ cm⁻³, which is approximately 8times as compared with the specific resistance of GaN of the samecarrier density, thereby increasing the operating voltage as well as thepower consumption in addition to the problem of heat generation.

Further, in a light emitting element of a semiconductor which uses aconventional semiconductor of gallium nitride type compound wherein theGaN layer is used as the contact layer 7 in which the p-side electrodeis to be made, due to such reasons that the GaN layer is affected byvariations of the surface level and that there is a large energy gapbetween the metallic conduction band such as the, alloy of Au or Au andZn used as electrode and the valence band of GaN, and the contactresistance between the electrode metal and the cap layer does notstabilize as a result, so that the contact resistance becomes large andthe operating voltage also rises. These problems results from a basicproblem that it is not possible to increase the carrier density of thep-type layer, and further, in a type of semiconductor laser with thecurrent injection area being restricted to stripe-like shape in whichthe contact area of the electrode is formed into a stripe-like shape,and the problem becomes more conspicuous.

Furthermore, as described above, the light emitting element of asemiconductor which uses a conventional semiconductor of gallium nitridetype compound is composed by laminating on a sapphire substrate asemiconductor layer of gallium nitride type compound by means of a lowtemperature buffer layer consisting of GaN and a high temperature bufferlayer, the lattice constant 4.758 Å of the sapphire substrate is largelydifferent from the lattice constant 3.189 Å of GaN, the interatomicbonding strength of GaN is strong although it is weaker than that ofAlGaN group, so that a crystal defect or transition is likely to occurdue to temperature shock. In case a crystal defect or transistion occursin the low temperature buffer layer, the crystal defect or transitionprogresses to the semiconductor layer formed thereon, therebydeteriorating the light emitting characteristic and reducing the life.

In addition, in the light emitting element of a semiconductor which usesa conventional semiconductor of gallium nitride type compound, asdescribed above, electric current flows between the p-side electrode 8provided on the contact layer 7 and the n-side electrode 9 provided onthe high temperature buffer layer 3 which is an n-type layer due to thevoltage applied therebetween, and the electric current flowing to then-side electrode 9 has high carrier density of the buffer layers 2 and3, so that the electric current flows throughout the buffer layers 3 and2. On the other hand, because the buffer layer, the low temperaturebuffer layer 2 in particular is formed on a substrate consisting, forexample, of sapphire that has a different lattice constant from that ofa semiconductor of gallium nitride type compound, crystal defects ortransition are likely to occur. When electric current flows into thebuffer layer where crystal defects or transition take place, crystaldefects or transition increase further due to the heat generated by theelectric current, such crystal defects or transition progress to thesemiconductor which contributes to the emission of light, therebylowering the light emitting characteristic, reliability or the life.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a light emitting device(hereinafter referred to as a light emitting element) of a semiconductorusing a highly characteristic semiconductor of gallium nitride typecompound which prevents increase in series resistance, that is, anincrease in operating voltage resulting from the use of a semiconductorof gallium nitride type compound as described above and restrictsfurther the occurrence of crystal defects or transition.

A first object of the present invention is to provide a light emittingelement of a semiconductor of double hetero structure in which the lightemitting efficiency does not lower with the operating voltage being low.

A second object of the present invention is to provide a semiconductorlaser of high light emitting efficiency which demonstrates sufficientlythe effect of confining the light in an active layer by a clad layereven in a semiconductor laser which uses a semiconductor of galliumnitride type compound and is suitable for emission of blue light, lowersthe series resistance of the semiconductor layer, and operates on lowoperating voltage.

A third object of the present invention is to p provide a light emittingelement of a semiconductor in which the contact resistance between then-side electrode and the contact layer is small and a large output canbe obtained from low operating voltage.

A fourth object of the present invention is to provide a light emittingelement of a semiconductor of high characteristic or efficiency and longlife which restricts occurrence of crystal defects or transition byreducing further the strain of the buffer layer on the surface of asubstrate consisting of sapphire and the like and prevents the progressof crystal defects or transition toward the semiconductor layer whichcontributes to emission of light.

The light emitting element of a semiconductor which realizes the firstobject of the present invention has at least a sandwich structureconsisting of an n-type clad layer, an active layer, and a p-type cladlayer, is light emitting element of a semiconductor of double heterojunction type which is formed by a material, band gap energy of saidactive layer being smaller than the band gap energy of both theforegoing clad layers, and both the foregoing clad layers being selectedso that the band gap energy of the foregoing n-type clad layer becomessmaller than the band gap energy of the foregoing p-type clad layer.

It is preferable that the foregoing n-type clad layer consists of ann-type Al_(x) Ga_(1-x) N (0≦x≦0.5), that the foregoing active layerconsists of In_(y) Ga_(1-y) N (0≦y≦1), that the foregoing n-type cladlayer consists of a p-type Al_(z) Ga_(1-z) N (0<z≦1), where 2x≦z.

It is preferable that a buffer layer consisting of GaN is providedbetween one of the foregoing clad layers and the substrate in order thatit is possible to relieve the strain of the clad layer, preventoccurrence of crystal defects or transition in the clad layer, and lowerthe resistance of the semiconductor layer.

In accordance with the light emitting element of a semiconductor whichrealizes the first object of the present invention, because a materialhaving smaller band gap energy than that of the p-type clad layer isused for the n-type clad layer, injection of electron into the activelayer from the n-type clad layer is carried out easily at low voltage.On the other hand, because a material having a large band gap energy isused for the p-type clad layer in the same way as in the past, escape ofelectron from the active layer to the p-type clad layer is less,contributing to recombination of electron and positive hole in theactive layer. Because the positive hole has a greater effective massthan the electron, there is less escape of the electron toward then-type clad layer side of the positive hole injected into the activelayer even if the band gap energy of the n-type clad layer is small.Therefore, the electron contributes to recombination in the active layerwithout wasting the positive hole, so that it is possible to lower theoperating voltage by the amount of the band gap energy of the n-typeclad layer is reduced, and the light of approximately the same luminanceis emitted.

Because the band gap energy of the n-type clad layer can be made smallis approximately three times the electron having effective mass of thepositive hole, it is possible to reduce the band gap energy of then-type clad layer to approximately half the difference of the band gapenergy between the p-type clad layer and the active layer, if the Al_(k)Ga_(1-k) N material is used, it is possible to reduce the ratio k of Alto less than half the ratio of Al of the p-type clad layer, and it ispossible to lower the operating voltage by 5 to 10%.

A light emitting element which realizes the second object of the presentinvention is a semiconductor laser of double hetero junction structurewhich has an active layer, an n-type layer and p-type layer consistingof a material having greater band gap energy and smaller refractiveindex that the active layer, and the foregoing active layer being heldbetween the foregoing n-type layer and the p-type layer, wherein theforegoing n-type layer and the p-type layer respectively consist of atleast two layers, a low refractive index layer consisting of materialwith small refractive index is provided on the foregoing active layerside of the n-type layer and p-type layer, and a low resistance layerhaving smaller electric resistance than the foregoing low refractiveindex layer is provided in other portion of an electric current path ofthe n-type layer and p-type layer.

It is preferable that the thickness of the foregoing low refractiveindex layer is 10 to 50% with respect to the thickness of the foregoingn-type layer or p-type layer, so that it is possible to confine thelight effectively in the active layer to certain extent and to lower theoperating voltage.

Similar to the foregoing, it is preferable that the thickness of theforegoing refractive index layer is 0.1 to 0.3 μm because it is possibleto confine the light effectively in the active layer to certain extentand to lower the operating voltage.

In order to obtain a semicondcutor laser which emits blue light ofexcellent light emitting characteristic, it is preferable that theforegoing active layer consists of Al_(m) Ga_(n) In_(1-m-n) N (0≦m<1,0<n<1, 0<m+n<1), the foregoing low refractive index layer consists ofAl_(r) Ga_(s) In_(1-r-s) N (0≦r<1, 0<s<1, m+n<r+s≦1, m<r), and theforegoing low resistance layer consists of Al_(t) Ga_(n) In_(1-t-u) N(0≦t<1, 0<u≦1, 0<t+u<1, m<t<r, m+n<t+u≦r+s).

It is preferable that m=0 in the material composition of the foregoingactive layer, that r+s=1 in the material composition of the foregoinglow refractive index layer, and that t=o and u=1 in the materialcomposition of the foregoing low resistance layer because it is possibleto obtain a semiconductor laser which is of more simple structure, hasexcellent light emitting characteristic, and emits blue light.

In accordance with the semiconductor laser which realizes the secondobject of the present invention, since the band gap energy forsandwiching the active layer is large, a low refractive index layer isprovided on the respective active layer side of the n-type layer andp-type layer consisting of a material with small refractive index, and alow resistance layer of small electric resistance is provided in anelectric current path, effect for confining the light into the activelayer is achieved by reflecting the light efficiently on the lowrefractive index layer, with respect to an increase in specificresistance due to the low refractive index layer, it is possible toreduce sufficiently the operating voltage by other low resistance layerwithout being influenced excessively by forming the low refractive indexlayer on a thin layer of approximately 0.1 to 0.3 μm for example.

As a result, it is possible to prevent heat generation due to excessiveresistance loss, improve the light emitting efficiency, and extend thelife.

The light emitting element of a semiconductor which realizes the thirdobject of the present invention is a light emitting element of asemiconductor in which a semiconductor layer of gallium nitride typecompound having a light emitting portion containing at least an n-typelayer and p-type layer is laminated on a substrate and an n-sideelectrode an p-side electrode to be connected respectively to theforegoing n-type layer and p-type layer are provided, and on at leastthe electrode side surface of the semiconductor layer to be providedwith the foregoing p-side electrode are provided a p-type In_(a)Ga_(1-a) N (0<a<1) or p-type GaSa or p-type GaP or p-type In_(b)Ga_(1-b) As (0<b<1) or In_(b) Ga_(1-b) P (0<b<1).

That the composition ratio of In of the foregoing In_(a) Ga_(1-a) N is0<a≦0.5 is preferable, so that it is possible to lower the contactresistance without emerging the problem of lattice mismatching.

In accordance with the light emitting element of a semiconductor whichrealizes the third object of the present invention, because In_(a)Ga_(1-a) N or GaAs or GaP or In_(b) Ga_(1-b) As or In_(b) Ga_(1-b) P isused on the surface of the contact layer to be provided with the p-sideelectrode, the contact resistance of the semiconductor layer and theelectrode becomes small. In other words, because these semiconductorlayer such as In_(a) Ga_(1-a) N has smaller band gap energy (forbiddenband width) that GaN and is difficult to be oxidized, it becomesdifficult for flowing electric current (contact resistance) due to thetrapping of electron or positive hole according to the surface level isreduced. Further, a semiconductor layer such as In_(a) Ga_(1-a) N hassmaller band gap energy (forbidden band width) as compared to GaN andthe energy gap between the energy level of the metallic conduction bandas an electrode and the valence band of the semiconductor layer issmall, permitting the positive hole to flow easily. There is a gapoccurring in the energy level of the valence band between In_(a)Ga_(1-a) N layer and GaN layer but the energy gap E_(v) between theelectrode metal and GaN layer is divided into the energy gap E_(v1)between the metal and In_(a) Ga_(1-a) N layer and the energy gap E_(v2)between In_(a) Ga_(1-a) N layer and GaN layer, so that the apparentcontact resistance becomes small since the positive hole or electronwhich has climbed over the small energy gap E_(v1) should climb over thesmall energy gap E_(v2) further whole it should not climb over the largeenergy gap E_(v) directly.

With respect to the GaAs pr GaP, because the band gap energy (forbiddenband width) is small and difficult to be oxidized in the same manner asIn_(a) Ga_(1-a) N, the surface level is difficult to occur and the bandgap energy is smaller than that of In_(a) Ga_(1-a) N, so that thecontact resistance becomes small further.

As in the case of the foregoing In_(a) Ga_(1-a) N, In of In_(b) Ga_(1-b)As or In_(b) Ga_(1-b) P plays a role of reducing the band gap energy(forbidden band width) further and acts to reduce the contact resistancefurther. In this case, lattice matching deviates largely from that ofIn_(a) Ga_(1-a) N but the effect of reduction of the band gap energy islarger. In addition, it is possible to increase the p-type carrierdensity further.

The light emitting element of a semiconductor which realizes the fourthobject of the present invention is a light emitting element of asemiconductor in which a semiconductor layer of gallium nitride typecompound having a luminous portion containing at least an n-type layerand a p-type layer is laminated on a substrate by means of a bufferlayer and at least the foregoing substrate side of the foregoing bufferlayer is consisted of a semiconductor layer of gallium nitride typecompound containing at least one kind of element selected from a groupconsisting of P and As.

It is preferable that the foregoing buffer layer has a low temperaturebuffer layer formed at least at low temperature and that the lowtemperature buffer layer is a semiconductor layer consisting of In_(c)Ga_(1-c) N (0<c<1) or In_(d) Al_(e) Ga_(n) In_(1-d-e) N (0<d<1, 0<e<1,0<d+e<1).

It is preferable that the foregoing buffer layer has a low temperaturebuffer layer formed at least at low temperature and that the lowtemperature buffer layer is a semiconductor layer consisting of GaN_(u)P_(1-u) (0<u<1) or GaN_(v) As_(1-v) (0<v<1) because it is possible toreduce the straining of the buffer layer.

In accordance with this light emitting element of a semiconductor,because In, P or As is contained in the buffer layer consisting ofsemiconductor of gallium nitride type compound on a sapphire substrate,the buffer layer becomes soft and occurrence of crystal defects ortransition becomes difficult. In other words, when a part of Ga of GaNbecomes In_(c) Ga_(1-c) N (0<c<1) which is substituted by In, In isheavier than Ga and is easily cut in a crystal, the strain is easilyrelieved, making it difficult for crystal defects and the like to occur.In addition, it becomes easy to form a polycrystalline film at lowtemperature by containing In, and it is possible to relieve the strainfurther by forming a buffer layer at low temperature. These phenomenaapply in the same manner to In_(d) Al_(e) Ga_(1-d-e) N (0<d<1, 0<e<1,0<d+e<1) of which part of Ga is substituted by Al.

Further, when part of N of GaN becomes GaN_(u) P_(1-u) (0<u<1) orGaN_(v) As_(1-v) (0<v<1) of which part of N of GaN is substituted by Por As, P or As becomes heavier than N and easily moves in a crystal.Therefore, according to the same reason that part of the foregoing Ga issubstituted by In, the strain of the buffer layer is relieved, so thatit becomes difficult for crystal defects or transition to occur.

Crystal defects or transition occurred in a buffer layer where thestrain is likely to occur most in contact with sapphire substrate andthe like progress toward a semiconductor layer which contributes to thelight emitting portion, and the strain of the buffer layer is relievedand the occurrence of crystal defects or transition of the buffer layeris restricted, so that the occurrence of crystal defects or transitionof a semiconductor layer which contributes to the light emitting portionis restricted, the light emitting characteristic is improved, and thelife and also improved.

Other light emitting element of a semiconductor which achieves the forthobject of the present invention consists of a semiconductor layer wherethe electric current is difficult to flow at least on the foregoingsubstrate side of the buffer layer.

That the foregoing buffer layer is a semiconductor layer of galliumnitride type compound containing at least one kind of element selectedfrom a group consisting of In, P and As is preferable because softnessis provided and it is possible to relieve the strain and to make itdifficult for crystal defects or transition to occur.

Because at least the foregoing substrate side of the foregoing bufferlayer is consisted of a semiconductor layer of high resistance, it ispossible to make it difficult for the electric current to flow.

Even in the case of a conductive type semiconductor layer which isdifferent from a conductive type semiconductor layer in which at lestthe foregoing substrate side of the foregoing buffer layer is laminateddirectly on the buffer layer, it is possible to make it difficult forthe electric current to flow to the substrate side of the buffer layer.

Because the foregoing buffer layer is formed at high temperature on ap-type low temperature buffer layer formed at low temperature on theforegoing substrate surface and such low temperature buffer layer,consisted of a high temperature buffer layer in which at least thesurface side is made an n-type, on such high temperature buffer layerare sequentially formed an n-type cald layer, an active layer, a p-typeclad layer, and a p-type contact layer in that order, the p-sideelectrode is formed on such p-type contact layer, the n-type electrodeis formed on the foregoing n-type clad layer exposed by etching or onthe high temperature buffer layer, it is possible to obtain a lightemitting element of a semiconductor of hetero junction structure inwhich it is difficult for the electric current to flow toward the bufferlayer on the substrate side.

Because the foregoing buffer layer consists of GaN, the foregoing n-typeand p-type clad layer consists of Al_(k) Ga_(1-k) N (0<k<1)respectively, the foregoing active layer consists of Ga_(y) In_(1-y) N(0<y≦1), and foregoing p-type contact layer consists of GaN, it ispossible to obtain a simply structured light emitting element of asemiconductor of double hetero junction structure.

In accordance with the light emitting element of a semiconductor of thepresent invention, because the semiconductor layer contacting at leastthe substrate of the semiconductor layer of gallium nitride typecompound to be laminated on the substrate is made a semiconductor layerwhere it is difficult for the electric current to flow, there will be nofurther increase in crystal defects or transition caused by the electriccurrent which occur due to the strain based on the difference of thelattice constant with respect to the substrate, and further the progressis restricted of crystal defects or transition toward the semiconductorlayer contributing to the light emitting portion, so that it is possibleto obtain a semiconductor layer of less crystal defects or transition.

In other words, when the electric current flows to a semiconductor layerwhere crystal defects or transition are occurring, a resistance lossoccurs and heat is generated in a portion where crysal defects ortransition are occurring, crystal defects or transition increasefurther, and the vicious cycle is repeated. On the other hand, becausecrystal defects or transition of the semiconductor of a portioncontributing to radiation results from the progress of crystal defectsor transition occurred in the semiconductor of a portion contributing toradiation results from the progress of crystal defects or transitionoccurred in the semiconductor layer contacting the substrate, byrestricting the occurrence of crystal defects or transition in thesemiconductor layer contacting the substrate, it is possible to restrictthe occurrence of crystal defects or transition in the semiconductorlayer which positively contributes to the emission of light. As aresult, a light emitting element of a semiconductor of excellent lightemitting characteristic, high reliability, and long life can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory drawing illustrating a section of an example 1of the light emitting element of a semiconductor of the presentinvention;

FIGS. 2(a) through 2(c) are explanatory drawings illustratingmanufacturing processes of FIG. 1;

FIG. 3 is an energy band drawing mainly illustrating a forbidden band ofa clad layer and an active layer of an example 1 of the light emittingelement of a semiconductor of the present invention;

FIG. 4 is an explanatory drawing of a section illustrating an example 2of the semiconductor laser of the present invention;

FIGS. 5(a) through 5(c) are explanatory drawings illustrating processesof the manufacturing method of an example 2 of the semiconductor laserof the present invention;

FIG. 6 is a drawing illustrating the relation of the light emittingefficiency with respect to the thickness of a low refractive indexlayer;

FIG. 7 is an explanatory drawing of a section illustrating an example 3of the light emitting element of a semiconductor of the presentinvention;

FIGS. 8(a) through 8(c) are drawings illustrating manufacturing methodsof the light emitting element of a semiconductor of FIG. 7;

FIGS. 9(a) through 9(b) are explanatory drawings of the energy band ofthe contact layer and the electrode metal of the light emitting elementof a semiconductor of FIG. 7;

FIGS. 10(a) through 10(d) are explanatory drawings illustratingmanufacturing processes of the example 4 of the light emitting elementof a semiconductor of the present invention;

FIG. 11 is a perspective drawing illustrating an example of aconventional light emitting element of a semiconductor and

FIG. 12 is an explanatory drawing of a section of a conventionalsemiconductor of GaAs type compound.

DETAILED DESCRIPTION

Referring now in detail to the drawings, the light emitting of asemiconductor of the present invention will be described.

EXAMPLE 1

FIG. 1 is an explanatory drawing of a section of a laser chip of amesa-type shape semiconductor of an example 1 of the light emittingelement of a semiconductor of the present invention, FIGS. 2(a) through2(c) are the manufacturing process drawing thereof, and FIG. 3 is aschematic illustration showing the energy band drawing mainly of theforbidden band of an n-type clad layer, an active layer, and a p-typeclad layer of the light emitting element of a semiconductor of theexample 1.

In FIG. 1, the numeral 1 indicates a substrate such as sapphire (singlecrystal of Al₂ O₃) consisting of an n-type GaN, wherein a lowtemperature buffer layer 2 of approximately 0.01 to 0.2 μm, a hightemperature buffer layer 3 of approximately 2 to 3 μm consisting of ann-type GaN, an n-type clad layer 4 of approximately 0.1 to 0.3 μmconsisting of an n-type material such as Al_(x) Ga_(1-x) N (0≦x≦0.5,where x=0.07 for example) which has a smaller band gap energy (forbiddenband width) than a p-type clad layer, an active layer 5 of approximately0.05 to 0.1 μm consisting of non-doping or an n-type or a p-typematerial such as In_(y) Ga_(1-y) N (0≦y≦1) which has a smaller band gapenergy and larger refractive index than both clad layers, a p-type cladlayer 6 of approximately 0.1 to 0.3 μm consisting of a p-type Al_(z)Ga_(1-z) N (0<z<1, 2x<z, where z=0.15 for example), and a contact layer(cap layer) 7 of approximately 0.3 to 2 μm consisting of a p-type GaNare laminated in order, a p-side electrode 8 consisting of a metal suchas Au is formed on the contact layer 7, an n-side electrode 9 consistingof a metal such as Al is formed on the high temperature buffer layer 3wherein a part of the laminated semiconductor layer is removed andexposed by etching, and a current stripe is formed, so that part of thecontact layer 7 and the p-type clad layer is etched and turned into amesa-type shape, and a chip of a semiconductor laser is formed.

In the light emitting element of a semiconductor of the presentinvention, as shown in an example of this semiconductor laser, the bandgap energy of the n-type clad layer 4 smaller than the band gap energyof the p-type clad layer 6, and both clad layers 4 and 6 are formed by amaterial having a greater band gap energy than that of the active layer5.

In order to recombine the electron and the positive hole efficiently andto improve the light emitting efficiency, a light emitting element of asemiconductor of double junction structure for sandwiching the activelayer 5 consisting of a material having a small band gap energy by cladlayers consisting of material having a large band gap energy, is usedfor a semiconductor laser or an LED of high luminance. When a materialhaving a large band gap energy is used for the clad layer, effect ofconfining the electron and the positive hole increases and contributesto the emission of light without waste but the operating voltageincreases, and in practice, a material having a band gap energy whichcan ignore to a certain degree the leakage of the electron and thepositive hole from the active layer, is selected. However, the operatingvoltage increases as compared with that of the pn junction. In thepresent invention, it is so designed that it is possible to lower theoperating voltage while maintaining this degree with which the electronand the positive hole can be ignored. In other words, the effective massof the positive hole is as approximately three times large as theeffective mass of electron, and the leakage from the positive hole issmaller than that from the electron even if the band gap energy issmall. For this reason, by using for the n-type clad layer a materialhaving a band gap energy smaller than that of the p-type clad layer, itis possible to inject the electron into the active layer at low voltageand prevent the leakage of the positive hole from the active layer.

Referring now to FIG. 3 which illustrates a schematic diagram showingthe energy band drawing of the foregoing semiconductor laser of FIG. 1,the action of the present invention will be described. In FIG. 3, theletter V represents the valence band, F the forbidden band, and C theenergy band of the conduction band respectively, and the letters Arepresents the high temperature buffer layer consisting of an n-typeGaN, B the n-type clad layer 4 consisting of n-type GaN, B the n-typeclad layer 4 consisting of an n-type Al₀.07 Ga₀.93 N, D the active layer5 consisting of In_(y) Ga_(1-y) N, G the p-type clad layer 6 consistingof Al₀.15 Ga₀.85 N, and J the contact layer 7 consisting of a p-type GaNrespectively of the ranges of the energy bands thereof.

In the semiconductor laser of this example, as shown in FIG. 3, the bandgap energy of the n-type clad layer indicated by B is formed to besmaller than the band gap energy of the p-type clad layer indicated byG. The band gap energy indicated by the broken line B₁ illustrates thecase of the band gap energy which is the same as the p-type clad layerin a conventional structure.

With this structure, when voltage is applied between the p-sideelectrode 8 and the n-side electrode 9, the electron E flows from then-type GaN (high temperature buffer layer A) to the p-side, and theninto the conduction band K₁ of the active layer. In this case, becausethe band gap energy of the n-type clad layer is low, the electron E islikely to flow into the conduction band K₁ of the active layer, so thatthe electron is supplied to the active layer even with low voltage. Theelectron E flowed into the conduction band K₁ of the active layer ispulled by the p-side electrode, but is confied in the active layerbecause the band gap energy of the p-type clad layer is large. On theother hand, the positive hole flows from the p-type GaN (contact layerJ) to the n-side, and then into the valence band K₂ of the active layer.The positive hole H flowed into the valence band K₂ of the active layeris pulled by the n-side electrode, but the effective mass of thepositive hole H is as approximately three times large as the effectivemass of electron, the positive hole cannot climb over the band gapenergy even if the band gap energy of the n-type clad layer B is low,and is confined effectively in the valence band of the active layer. Asa result, recombination of the electron and the positive hole is carriedout in the active layer efficiently, and high light emitting efficiencycan be obtained.

As described above, in accordance with the present invention, becauseeach semiconductor layer is selected so that the band gap energy of then-type clad layer is made smaller than that of the p-type clad layer, itis possible to inject the electron into the active layer with lowvoltage and improve the light emitting efficiency without increasing thereactive current. The amount that the band gap energy of the n-type cladlayer is smaller than that of the p-type clad layer is determined by theband gap energy of the active layer, and the amount may be as low as 1/3to 1/2 in the case of the p-type layer by the difference of the band gapenergy of the active layer.

In order to reduce the band gap energy by using a semiconductor ofgallium nitride type compound consisting of a general formula Al_(p)Ga_(1-p-q) N (0≦p<1, 0<q≦1, 0<p+q≦1), it is possible to obtain a smallband gap energy by making p small, that is, by making the compositionratio of Al small, or by making p+q small, that is, by making thecomposition ratio of In large. For this reason, by adjusting thecomposition ratio of Al and In so as to make the band gap energy of theclad layer larger than that of the active layer, and so as to make theband gap energy of the r-type clad layer smaller than that of the p-typeclad layer, it is possible to obtain a semiconductor layer of a desiredband gap energy.

Because the example shown in FIG. 1 refers to a semiconductor, it isnecessary to confine the light in the active layer and oscillate it, sothat the refractive index of the clad layer is made smaller than that ofthe active layer, but it is not always necessary to do so in the case ofLED. However, if the composition ratio of Al is made large by theforegoing composition ratio, the refractive index is made small.

Next, by referring to FIGS. 2(a) through 2(c), the manufacturing methodof the semiconductor laser shown in FIG. 1 will be described.

First, as shown in FIG. 2(a), on a substrate 1 consisting of sapphireand the like is grown by the MOCVD method, approximately 0.01 to 0.2 μmof a low temperature buffer layer 2 consisting, for example, of asemiconductor of gallium nitride type compound such as the n-type GaN,and approximately 2 to 5 μm of a high temperature bufrer layer 3consisting of the n-type GaN of the same composition is formed at 700°to 1200° C.

Next, TMI is supplied further, and approximately 0.1 to 0.3 μm of ann-type clad layer 4 consisting, for example, of the n-type Al_(x)Ga_(1-x) N (0≦x≦0.5, where x=0.07 for example) is formed. Afterward, TMIis supplied instead of TMA and an active layer 5 consisting of anon-doping type or an n-type or p-type In_(y) Ga_(1-y) N (0≦y≦1, wherey=0.06 for example) is caused to grow to a thickness of approximately0.05 to 0.1 μm. Then, by using the same material gas as the material gaswhich is used to form the n-type clad layer 4, TMA is supplied to areaction tube at the flow rate of 20 to 100 sccm which is approximatelytwice the case of the n-type clad layer 4 so as to form approximately0.1 to 0.3 μm of the p-type Al_(z) Ga_(1-z) N (0<z≦1, 2x≦z, where z=0.15for example) which is the p-type clad layer 6. Further, the samematerial gas used for forming the buffer layer 3 is supplied so as toform approximately 0.3 to 2 μm of a contact layer 7 consisting of thep-type GaN.

In order to form the foregoing buffer layer 3 or clad layer 4 into ann-type clad layer, Si, Ge, Tc is mixed into a reaction oven as theimpurity material gas such as SiH₄, GeH₄, and TeH₄, and in order to formthe clad layer 6 or the contact layer 7 into a p-type clad layer, Mg orZn is mixed into the material gas as the organometallic gas of Cp₂ Mg orDMZn. However, in the case of the n-type, even if the impurity is notmixed, N is easily evaporated during film forming and is turned into then-type naturally, and therefore such nature may be utilized.

Afterwards, a protective film 10a such as SiO₂ or Si₃ N₄ is providedover the entire surface of the grown layer of a semiconductor layer(refer to FIG. 2(b)), annealed for approximately 20 to 60 minutes at 400to 800° C., and the p-type clad layer 6 and the cap layer 7 which arethe p-type layer.

When annealing is completed, as shown in FIG. 2(C), a mask such as aresist film 10b is provided and the laminated semiconductor layer isetched until the n-type clad layer 4 or the n-type high temperaturebuffer layer 3 is exposed. This etching is carried out by the reactiveion etching under the atmosphere of a mixed gas of Cl₂ and BCl₃ forexample.

Then, a metallic film such as Au and Al is formed by sputtering, on thesurface of the laminated compound of semiconductor layer is formed thep-side electrode 8 to be electrically connected to the p-type layer, onthe surface of the exposed high temperature buffer layer 3 is formed then-side electrode 9 to be electrically connected to the n-type layer, andpart of the contact layer 7 and the p-type clad layer 6 are etched intomesa-type shape layers (refer to FIG. 1).

Next, each chip is diced, and thus semiconductor laser chips are formed.

In this example, the semiconductor laser of mesa-type shape currentstripe structure is described, but the present invention can also beapplied to a semiconductor laser of various structures such as the flushtype current limiting layer or the light emitting element ofsemiconductor which uses a semiconductor of gallium nitride typecompound such as LED of double hetero junction structure.

In accordance with the light emitting element of a semiconductor of thisexample, because the semiconductor material is selected so that it ispossible to make the band gap energy of the n-type clad layer smallerthan the band gap energy of the p-type clad layer, reactive current isless, and, it is possible to emit the light of high luminance with lowoperating voltage and to obtain a light emitting element ofsemiconductor having high light emitting efficiency.

EXAMPLE 2

FIG. 4 is an explanatory drawing of a section of the chip of asemiconductor laser which is the other example of the light emittingelement of the semiconductor of the present invention, and FIGS. 5(a)through 5(c) are an explanatory drawing of a section of the process ofthe manufacturing method thereof.

As shown in FIG. 4, in the semiconductor laser of this example, a lowbuffer layer 22 of approximately 0.01 to 0.2 μm which is a lowerresistance layer with small electric resistance consisting of an n-typeAl_(t) Ga_(1-t-u) N (0≦t<1, 0<u≦1, 0<t+u≦1) and the like provided on asubstrate 1 such as sapphire (single crystal of Al₂ O₃), a hightemperature buffer layer 23 of approximately 2 to 5 μm, an n-type cladlayer 24 of approximately 0.1 to 0.3 in which is low refractive indexlayer consisting of an n-type Al_(r) Ga_(s) In_(1-r-s) N (0≦t<r<1,0<s<1, 0<r+s<1) and having the refractive index smaller than that of theforegoing buffer layers 22 and 23, an active layer 25 of approximately0.05 to 0.1 μm consisting of Al_(m) Ga_(n) In_(1-m-n) N (0<m<r,0<m+n<r+s) of a non-doping type or an n-type or a p-type and having asmaller band gap energy and a larger refractive index than the n-typeclad layer 24 (the ratio n of Al is small and the ratio 1-m-n of In islarge), a p-type clad layer 26 of approximately 0.1 to 0.3 μm which is alow refractive index layer of the same composition as the n-type cladlayer 24, a current limiting layer 20 consisting of GaN and the likeformed with a stripe groove, and a cotnact layer consisting of thep-type Al_(t) Ga_(u) In_(1-t-u) N (0≦t<1, 0<u≦1, 0<t+u≦1) ofapproximately 3 to 2 μm which is a low refractive index layer of thesame composition as the buffer layers 22 and 23 are laminated in order,the p-side electrode 8 is provided on the surface of the contact layer7, the n-side electrode 9 on the high temperature buffer layer 23 whichis exposed by etching a part of the laminated semiconductor layer, and achip of the semiconductor laser of this example is formed.

In the semiconductor laser of this example, the n-type layer and thep-type layer which hold the active layer 25 is separated into the bufferlayers 22 and 23 which are the low resistance layer respectively, then-type clad layer 24 which is the low refractive index layer, the p-typeclad layer 6 which is the low refractive index layer, and the contactlayer 27 which is the low resistance layer, and the low refractive indexlayer is formed into a minimum thickness necessary to confine the lightinto the active layer. In order to inspect the influence on the lowrefractive index layer by the thickness, the inventor of the presentinvention inspected the light emitting efficiency in a manner as shownin an example of concrete structure to be described layer, namely, onthe buffer layer 23 is formed approximately 3 μm of the n-type GaN, onthe n-type and p-type clad layers 24 and 26 are formed Al₀.3 Ga₀.7 N,and on the contact layer 27 is formed approximately 2 μm of the p-typeGaN respectively, the thickness of the clad layers 24 and 26 are changeddifferently from 0 to 0.4 μm (the thickness of the buffer layer 3 andthe contact layer 27 is also changed every time so as to keep the totalthickness of the clad layers 24 and 26 constant), and thus the lightemitting efficiency is inspected. Results of the inspection is shown inFIG. 6. As is apparent from FIG. 6, it is necessary that the thicknessof the low refractive index layer is formed to be 0.05 to 0.3 μm, andpreferably to be approximately 0.1 to 0.2 μm, and is formed to be 10 to50% with respect to the entire thickness of the n-type layer or thep-type layer, and preferably to be approximately 10 to 30%.

In case where a semiconductor of gallium nitride type compound isexpressed as the general formula of Al_(p) Ga_(q) In_(1-p-q) N, in orderto make the refractive index small in a semiconductor of gallium nitridetype compound, it is necessary to make the composition ratio of Al largeand to make the composition ratio of In small, and when the compositionratio of Al is made large, the resistance is increased and theresistance loss results. Therefore, in this example, a low refractiveindex layer (the n-type clad layer 24 and the p-type clad layer 26) isprovided for confining the light into the semiconductor portion adjacentto the active layer 25 of the n-type layer (the buffer layers 22 and 23,and the n-type clad laeyr 24) which hold the active layer 25therebetween, and other portion is made a low resistance where the lossof resistance does not occur.

In order to make the specific resistance small in the semiconductor ofgallium nitride type compound expressed as the foregoing general formulaof Al_(p) Ga_(q) In_(1-p-q), the composition ratio of Al is to be simplymade small. In this case, it is necessary for the n-type layer and thep-type layer which hold the active layer therebetween to be providedwith the function as the clad layer to confine the light into the activelayer, it is preferable that even the low resistance layer (the bufferlayers 22 and 23 and the contact layer 27) is formed by a material whichhas smaller refractive index and larger band gap energy than the activelayer 25. In order to achieve this object, by making the compositionratio of In large in the active layer 25 and by making such compositionratio small in the n-type layer and the p-type layer, it is possible toobtain a layer which is of low resistance and has larger band gap energyand smaller refractive index than the active layer 25.

In this example, the composition ratios of the n-type buffer layers 22and 23 and the contact layer 27 are made to be the same, and thecomposition ratios of the n-type clad layer 24 and p-type clad layer 26are made to be the same, but respective composition ratios may notalways have to be the same, and the low refractive index layer (then-type clad layer 24 and p-type clad layer 26) may have smallerrefractive index and larger band gap energy than the active layer 25 orthe low resistance layer (the buffer layers 22 and 23 and the contactlayer 27), and the low resistance layer may be of a material havingsmaller specific resistance than the low refractive index layer.

The low temperature buffer layer 22 is made to be a low resistance layerin this example, but it is not necessary to make the low temperaturebuffer layer to be an electric current path if it is possible to providethe n-side electrode 9 on the high temperature buffer layer 23, so thatthe low temperature buffer layer 22 may be made to be a high resistancelayer or the p-type layer. In this case, it is preferable that electriccurrent is not flowed to the low temperature buffer layer where crystaldefects or transition are likely to occur, thereby making it possible toprevent an increase of crystal defects or transition.

Further, in the boundary between the low resistance layer and the lowrefractive index layer, the composition may not be changed sharply butmay be changed gradually. Causing the composition to change gradually iseffective to restrict the strain generating in the interface due tosudden change in composition. In addition, a second clad layer which isa low resistance layer similar to the contact layer 27 may be providedbetween the p-type clad layer 26 and the contact layer 27, so that thehigh temperature buffer layer 23 accomplishes the function of the cladlayer of low resistance layer.

Next, the manufacturing method of the semiconductor laser of thisexample will be described by a definite example thereof. First, as shownin FIG. 5(a), a substrate 1 consisting of sapphire and the like isinstalled in a reaction tube, and in the same manner as the example 1,10 slm of the carrier gas H₂, 100 sccm of the reactant gas TMG, and 10slm of NH₃ are introduced to grow in vapor phase at 400° to 700° C. bymeans of the organometallic vapor phase growth method (hereinafterreferred to as the MOCVD method), and a low temperature buffer layer 22which is a polycrystalline film consisting of GaN of approximately 0.01to 0.2 μm is formed. Then, by raising the temperature to 700° to 1200°C. and allwoing to stand for approximately 5 to 15 minutes, polycrystalline of the low temperature 5 to 15 minutes, polycrystal line of the lowtemperature buffer layer 2 is made into a single crystal, by introducingthereon the same material gas as the foregoing and causing it to reactin vapor phase at high temperature of 700° to 1200° C., a hightemperature buffer layer 23 consisting of single crystal of GaN isformed to 2 to 5 μm thick.

Further, by mixing TMA at the flow rate of 10 to 200 sccm and causing itto react in vapor phase, an n-type clad layer 24 consisting of Al₀.2Ga₀.8 N is formed to 0.1 to 0.3 μm thick.

Next, the dopant SiH₄ is stopped and in place of TMA, TMI is supplied atthe flow rate of 10 to 200 sccm so as to form approximately 0.05 to 0.1μm of a non-doping active layer 5 consisting of In₀.1 Ga₀.9 N, furtherthe material gas of the same composition as that of the n-type cladlaeyr 24 is supplied, the impurity material gas is replaced by SiH₄ andCp₂ Mg or DMZn is introduced at the flow rate of 10 to 1000 sccm so asto form the p-type clad layer 6 consisting of Al₀.2 Ga₀.8 N to 0.1 to0.3 μm thick, and then, the n-type GaN layer is formed to approximately0.1 to 0.5 μm thick so as to form a current limiting layer 20 bysupplying TMG, NH₃, and SiH₄.

Afterward, the furnace temperature is lowered to approximately the roomtemperature, a substrate with semiconductor layer laminated thereon istaken out from the MOCVD device, and as shown in FIG. 5(b), a stripegroove is formed by etching by lithographic process, and the currentlimiting layer 20 is formed.

Afterwards, as shown in FIG. 5(c), the substrate is put into the MOCVDdevice again with the temperature set at 700° to 1200° C., TMG and NH₃as the reactant gas and Cp₂ Mg or DMZn as the dopant are supplied, andapproximately 2 to 3 μm of a contact layer 27 consisting of GaN isformed.

Afterwards, a protective film such as SiO₂ and Si₃ N₄ is provided on theentire surface of a semiconductor layer and annealed for approximately20 to 60 minutes at 400° to 8000° C. so as to activate the p-type cladlayer 26 and the contact layer 27.

Next, in order to form the n-side electrode, a mask is formed by aresist film and the like and part of a semiconductor layer laminatedunder atmosphere of Cl₂ gas is provided with reactive ion etching, ahigh temperature buffer layer 23 which is an n-type layer is caused tobe exposed, a p-side electrode 8 consisting of Au and Au--Zn and thelike is formed on the contact layer 27, an n-side electrode 9 consistingof Au and Au--Zn and the like is formed on the contact layer 27, ann-side electrode 9 consisting of Al and Au--Gc and the like is formed onthe high temperature buffer layer 23, and a chip of a semicondcutorlaser is formed by dicing (refer to FIG. 4).

In this example, the semiconductor laser is a semiconductive laser ofrefractive index wave guide type provided with the current limitinglayer 20, but it is also the same in the case of a gain wave guidestripe type semiconductor laser. Further, it is the semiconductor laserwhich uses a semiconductor of gallium nitride type compound whosegeneral formula is expressed as Al_(p) Ga_(q) In_(1-p-q) N, and thoughit is not so remarkable as the semiconductor of gallium nigride typecompound, it will be effective by application of the present inventioneven in the case of a semiconductor of other compound such as thesemiconductor of arsenic gallium type compound.

In accordance with a semiconductor laser of the example 2, because then-type layer and the p-type layer for holding the active layertherbetween are respectively formed by at least a low refractive indexlayer and a low resistance layer, confinement of the light into theactive layer is carried out by the low refractive index layer, electricresistance of the portion composing other electric current path becomessmall due to the low resistance layer, so that it is possible to lowerthe operating voltage. As a result, it is possible to obtain asemiconductor laser which operates with low operating voltage and hashigh light emitting efficiency.

EXAMPLE 3

In a further other example of the semiconductor laser of the presentinvention shown in FIG. 7, a low temperature buffer layer 2 ofapproximately 0.01 to 0.2 μm consisting of the n-type GaN and the likeon a substrate 1 such as sapphire (single crystal of Al₂ O₃), a hightemperature buffer layer 3 of approximately 2 to 5 μm, a lower caldlayer 4 of approximately 0.1 to 0.3 μm consisting of the n-type Al_(k)Ga_(1-k) N (0<k<1), an active layer 5 of approximately 0.05 to 0.1 μmconsisting of a non-doping or n-type or p-type In_(y) Ga_(1-y) N (0<y<1)and having smaller band gap energy and larger refractive index that thelower clad layer 4, an upper clad layer 6 of approximately 0.3 to 2 μmwhich is of the same composition as the lower clad layer 4 and of thep-type, and a contact layer 37 consisting of the p-type GaN layer 37a ofapproximately 0.3 to 2 μm which is of the same composition as the bufferlayers 2 and 3 and of the p-type and the p-type In_(a) Ga_(1-a) N(0<a<1) layer 27b of approximately 0.05 to 0.2 μm provided on thesurface thereof are laminated in order, a p-side electrode 8 is providedon the In_(a) Ga_(1-a) N layer 37b of the surface of the contact layer37, an n-side electrode 9 is provided on the n-type clad layer 4 or thehigh temperature buffer layer 3 exposed by etching a part of thelaminated semiconductor layer, and a chip of the semiconductor laser ofthis example is formed.

In the semiconductor laser of this example, a semiconductor layer ofgallium nitride type compound is laminated, the material of the p-typecontact layer 37 to be provided with the p-type electrode 8 has smallerband gap energy than GaN and is difficult for the surface level tooccur, for example, the p-type In_(a) Ga_(1-a) N layer 37b is providedon the surface of the GaN layer 37a, and the p-side electrode 8 isprovided on the In_(a) Ga_(1-a) N layer 37b. Since lattice mismatchingresults if In is mixed with GaN, such mixture is not used in a layerexcept when a material of small band gap energy is indispensable as inthe case of the active layer, and there has been no conception at all touse the In_(a) Ga_(1-a) N layer in the contact layer 37. However, in thelight emitting element of a semiconductor which uses a semiconductor ofgallium nitride type compound, it is not possible to increase thecarrier density of the p-type layer above a certain value, and theincrease in the contact resistance between the p-type layer and thep-side electrode increased the operating voltage and caused loweredlight emitting efficiency. As the reault of assiduous studies, theinventor of the present invention overcome the problem of latticemismatching by forming the semiconductor layer to a thickness ofapproximately 0.05 to 0.2 μm in which a film is formed even if a littlelattice mismatching should occur, found out that it is possible to lowerthe contact resistance with metal, and completed the present invention.

In the case where In_(a) Ga_(1-a) N contained with In as the material ofsmall band gap energy is used, if the composition ratio of In isincreased, the thickness for example of the foregoing semiconductor wasnot preferable in that a phenomenon such as occurrence of transitionemerged, but by setting the composition ratio a of In to 0<a≦0.5,preferable to 0.05≦a<0.3, more preferable to 0.05≦x≦0.2, it was madepossible to reduce the contact resistance with metal without causing theproblem transition.

Even in the case where GaAs or Gap was used as the material of smallband gap energy instead of In_(a) Ga_(1-a) N, it was possible to performan operation in which the contact resistance with metal was also small,it was also difficult for the surface level to occur on thesemiconductor surface, and the operating voltage was low. Growthtemperature of GaAs or GaP is different from that of the GaN type layer,but it is possible to obtain a layer by forming the GaN type layer andthen by growing it by lowering the temperature inside the MOCVD deviceto 500° to 800° C. Though lattice mismatching of GaN occurs with respectto GaAs or GaP, but the influence due to lattice mismatching is reducedto certain extent by setting the foregoing thickness to approximately0.05 to 0.2 μm.

By mixing In further with GaAs or GaP, it is possible to use thecharacteristic that it is easy to make alloy using a metal which isdifficult to be oxidized than Al and Ga, and it was possible to reducethe contact resistance further. In this case, it is possible to increasethe composition ratio of In to approximately 0 to 0.5.

Next, referring to FIG. 9(a) and FIG. 9(b), and by providing asemiconductor of small band gap energy on the surface of the contactlayer 37, the principle that the contact resistance with the p-sideelectrode is reduced will be described.

FIG. 9(a) and FIG. 9(b) are the drawings showing the energy band of thecontact layer 37 and the p-side electrode 8, in which the left side ofthe drawing indicates the side of the p-type clad layer 6 of the contactlayer 37, the right thereof indicates the side of the p-side electrode8, and the letter L represents the energy band of the GaN layer 37a, Mrepresent that of the In_(x) Ga_(1-x) N, layer 37B, and N represent thatof part of the p-side electrode 8 respectively. FIG. 9(a) and FIG. 9(b)typically illustrate the state where the energy levels move upward anddownward on the surface of the GaN layer 37a or the In_(a) Ga_(1-a) Nlayer 37b depending on the composition of the semiconductor layer andthe kind of the metal for electrode to be provided on the surface of thesemiconductor layer, and indicate the same phenomena in any state ofFIG. 9(a) and FIG. 9(b). In FIG. 9(a) and FIG. 9 (b), P₁ and P₂ indicatethe valence band of GaN and In_(a) Ga_(1-a) N respectively, Q₁ and Q₂the conduction band respectively, and R the energy level where theelectron of an electrode metal is maximum. The gap F₁ of P₁ and Q₁ andthe gap F₂ of P₂ and Q₂ respectively represent the band gap energy(forbidden band) of GaN and In_(a) Ga_(1-a) N. The flow of electriccurrent from the p-side electrode to the contact layer means that thepositive hole moves from the energy level R of the electrode metal tothe valence band P₁ of GaN, but in accordance with the presentinvention, because the In_(a) Ga_(1-a) N layer is provided, it is normalonce the positive hole climbs over the gap energy E_(v1) and flows tothe valence band P₂ of the In_(a) Ga_(1-a) N layer, and then climbs overthe gap evergy E_(v2) from P₂ to P₁ and flows, so that the positive holeflows easily because it is not neecssary for the positive hole to climbover the gap energy E_(v) at one time when there is no In_(a) Ga_(1-a)N. In this case, assuming that the constant of the electric currentincluding the item of the temperature and k₁ and k₂ respectively, thegap energy can be expressed by exp{-(k₁ E_(v1) +k₂ E_(v2))]. The reasonthat the energy barrier is devided into two stages is that the forbiddenband width F₂ of In_(a) Ga_(1-a) N is smaller than the forbidden bandwidth F₁ of GaN, and ideally, it is desirable to use the material of theforbidden band width F₁ of GaN GaAs or GaP or In_(b) Ga_(1-b) As orIn_(b) Ga_(1-b) P has also the same relation of the forbidden bandwidth, and the contact resistance is reduced in the same manner.

Next, referring to FIGS. 8(a) through 8(c), the manufacturing method ofthe semiconductor laser of this example shown in FIG. 7 will bedescribed.

First, as shown in FIG. 8(a), to the substrate 1 consisting of sapphireand the like in the same manner as the example 1 is supplied by theMOCVD method the carrier gas H₂ together with TMG and NH₃ which are theorganometallic compound gas, and SiH₄ as the dopant, and the lowtemperature buffer layer 2 and the high temperature buffer layer 3consisting of gallium nitride type semiconductor layer such as then-type GaN layer are grown respectively to approximately 0.01 to 0.2 μmand 2 to 5 μm.

Then, TMA is added further to the foregoing gas, and n-type clad layer 4containing Si and the like of the n-type dopant as the SiH₄ gas and thelike is formed to approximate thickness of 1 to 2 μm.

Next, as a material in which the band gap energy is smaller than that ofthe clad layer, in place of the foregoing material gas for example, TMIis introduced and approximately 0.05 to 0.1 μm of an active layer 5 isformed, further, in place of SiH₄, the same material gas used forforming the n-type clad layer 4 is used and Cp₂ Mg or DMZn is introducedinto a reaction tube as the p-type impurity, and the p-type GaN layerwhich is the p-type clad layer 6 is caused to grow in vapor phase.

Then, as shown in FIG. 8(b), in order to form a contact layer, Cp₂ Mg orDMZn is supplied as the dopant gas using the same gas of the foregoingbuffer layer 3, and the p-type GaN layer 37a is grown to a thickness ofapproximately 0.3 to 2 μm.

Further, in order to reduce the contact resistance with the p-sideelectrode, TMI is added to the same material gas as the foregoing GaNlayer 37a, and the In_(a) Ga_(1-a) N (0<a<1, where a=0.1 for example) isformed to a thickness of approximately 0.05 to 0.2 μm. If the In_(a)Ga_(1-a) N layer is excessively thick, the resistance of the film itselfinfluences the entire layer, and if it is excessively thin, the contactresistance cannot be reduced.

In the foregoing description, the p-type In_(a) Ga_(1-a) N layer is usedas part of the contact layer, but by changing the gas to the p-typeIn_(a) Ga_(1-a) N, it is possible to obtain the same effect by formingthe p-type GaAs, p-type GaP, p-type In_(b) Ga_(1-b) As (0<b<1, whereb=0.2 for example) or p-type In_(b) Ga_(1-b) P (0<b<1, where b=0.5 forexample) as the contact layer on the side contacting the p-sideelectrode. In this case, it is possible to obtain the same effect bylowering the inside temperature of the MOCVD device to 500° to 800° C.and introducing the gas, in place of the foregoing TMI, or introducingTMI and tertiary butyl arsine (TBA) or tertiary butyl phosphine (TBP).

Afterwards, a protective film such as SiO₂ and Si₃ N₄ is provided overthe entire surface of the grown layer of a semiconductor, annealing orelectron irradiation is provided for approximately 20 to 60 minutes at400° to 800° C. so as to activate the p-type clad layer 6 and thecontact layer 37. Upon completion of annealing, the protective film isremoved by wet etching.

Then, in order to form an electrode on the n-side, resist is applied andpatterning is provided, a resist film is provided on the surface of asemiconductor layer of gallium nitride type compound from which theprotective film is removed as shown in FIG. 8(c), part of thesemiconductor layer is removed by dry etching, the high temperaturebuffer layer 3 which is an n-type layer or the n-type clad layer 4 isexposed, on the surface of the exposed high temperature buffer layer 3(or the n-type clad layer 4) is formed an n-side electrode 9 consistingof Al and the like to be electrically connected to the n-type layer, andon the surface of the contact layer 37 of a semiconductor layer of thelaminated compound is formed a p-side electrode 8 consisting of ametallic film such as Au and Zn respectively by sputtering and the like.Next, part of the contact layer 7 and the p-type clad layer is turnedinto the mesa-type shape by etching, and a semiconductor laser chip isformed by dicing each chip.

And, a structure for lowering the contact resistance between the p-sideelectrode 8 and the contact layer 37 of this example can be applied tothe light emitting element of various semiconductors such as an LED ofdouble hetero junction or an LED of pn junction.

In accordance with the light emitting element of the semiconductor ofthe example 3, because the portion at least contacting the p-sideelectrode of the contact layer of the p-side electrode is formed with asemiconductor material having smaller band gap energy than the p-typeGaN, it is possible to reduce the influence of the surface level and itis also possible to reduce the contact resistance with the p-sideelectrode. Therefore, it is possible to reduce the operating voltage andimprove the light emitting efficiency.

EXAMPLE 4

In the light emitting element of the semiconductor of the example, whenforming a semiconductor of gallium nitride type compound on a substratesuch as sapphire, a semiconductor layer in which In, P or As iscontained is provided on a semiconductor layer at least contacting asubstrate, the strain of such semiconductor layer is relieved so as torestrict occurrence of crystal defects or transition involved in latticemismatching with a substrate such as sapphire, thereby preventing theprogress of crystal defects or transition toward the semiconductor layerwhich contributes to the light emitting portion.

A semiconductor layer containing In is one of which part of GaN isreplaced by In and by forming on the substrate as a semiconductor layerof In_(c) Ga_(1-c) N (0<c<1), In becomes heavier than Ga and is movedeasily, so that it is possible to form a soft buffer layer with lessstrain in which coupling between atoms is easily cut. The compositionratio c of In is set to 0 to 1, and preferable to 0.1 to 0.5, and morepreferably to approximately 0.1 to 0.3. If the composition ratio of Inis excessively large, the difference between the substrate and thelattice constant becomes excessively large, posing excessively largeproblem of lattice mismatching, and therefore, the effect of relievingthe strain by In is now shown if the composition ratio of In isexcessively small. In this In_(c) Ga_(1-c) N the same applies to thecase of In_(c) Al_(e) Ga_(1-d-e) N (0<d<1, 0<e<1, 0<d+e<1) in which partof Ga is replaced further by Al.

And, in a semiconductor layer containing p or As, part of N of GaN isreplaced by P or As and formed on a substrate as GaN_(w) P_(1-w) (0<w<1)or GaN_(v) As_(1-v) (0<v<1), because p or As is heavier than N and ismoved easily, so that it is possible to form a soft buffer layer withless strain in which coupling between atoms is easily cut. Compositionratio w and V of P and As is set to 0<w and v≦0.2, preferably to 0<w andv≦0.1, and more preferably to approximately 0.02≦w and v≦0.06. When thecomposition ratio of P and As becomes excessively large, latticemismatching between GaN and N becomes large, and therefore, the effectof relieving the strain is low shown if the composition ratio of P andAs is excessively small.

In P and As, both atoms may be contained not only in one side only butmay be contained in to form of mixed crystals. In this case, it ispreferable that the total composition ratio of P and As is contained inthe range of the foregoing w or v. Further, each atom such as In and P,In and As, and In and As may form a mixed crystal respectively. In thiscase, it is possible to contain in the range of the foregoing c and tocontain P and/or As in the range of foregoing w or v.

Next, further details will be described in concrete example.

FIGS. 10(a) through 10(d) are explanatory drawings of a section of theprocess of the concrete example of the light emitting element of asemiconductor of the example 4.

First, as shown in FIG. 10(a), to a substrate 1 consisting of sapphireand the like is supplied by the MOCVD method the carrier gas, TMG, NH₃,and TMI, a low temperature buffer layre 42 consisting of a semiconductorlayer of the n-type gallium nitride type compound of In_(c) Ga_(1-c) N(0<c<1, where c=0.2 for example) is formed to approximate thickness of0.01 to 0.2 μm at low temperature of 400° to 600° C . Composition ratioof In is set preferably to 0.1 to 0.5, sand more preferably to 0.1 to0.3. If the composition ratio is excessively large, the problem oflattice mismatching results and if excessively small, the effect ofrelieving the strain is not shown.

Afterwards, in order to form a buffer layer of single crystal,temperature is at 700° to 1200° C. and the low temperature buffer layeris made into a single crystal layer, and on the surface thereof is grownthe high temperature buffer layer 3 consisting of GaN or AlGaN type orInAlGaN type and the like to approximate thickness of 2 to 5 μm.Afterwards, a clad layer 4 consisting, for example, of the n-type Al_(k)Ga_(1-k) N (0<k<1) and active layer 5 consisting of In_(y) Ga_(1-y) N(0<y<0.2) is grown to approximate thickness 0.1 to 0.3 μm and 0.05 to0.1 μm respectively. To form the clad layer, TMG, NH and TMA, and SiH₄as the dopant are supplied, and TMI is introduced in place of TMA andcaused to react.

Further, with the same material gas as the material gas used for formingthe n-type clad layer 4, impurity material gas is replaced by SiH₄, Cp₂Mg or DMZn gas is introduced into a reaction tube as the p-type impuritymaterial gas, so that the p-type Al_(k) Ga_(1-k) N (0<k<1) layer whichis the p-type clad layer 6 is caused to grow in vapor phase.

Then, a contact layer (cap layer) 7 consisting, for example, of thep-type GaN layer is grown and formed to 0.2 to 3 μm.

Afterwards, as shown in FIG. 10(c) in the same manner as the example 1,a protective film 10a such as SiO₂ and Si₃ N₄ is provided on the entiresurface of the grown layer of a semiconductor layer, annealing orelectron irradiation is provided for approximately 20 to 60 minutes at400° to 800° C., thereby activating the p-type clad layer 6 and thecontact layer 7.

Then, in order to form an electrode of the n-side after the protectivefilm 10a is removed, resist is applied and patterning provided, a cladlayer 4 or buffer layer 3 which are the n-type layer are exposed (referto FIG. 10(c)) by removing part of each of grown semiconductor layer byetching in the same manner as the example 1, the n-side electrode 9 tobe electrically connected to the n-type layer and the p-side eletrode 8to be electrically connected to the p-type contact layer 7 arerespectively formed on the surface of a semiconductor layer of thelaminated comopund by sputtering and the like (refer to FIG. 10(d)), andLED chips are formed by dicing.

In accordance with this example, because a semiconductor layer ofgallium nitride type compound containing In is used for the lowtemperature buffer layer 42 as the semiconductor layer which contacts asubstrate such as sapphire, the semiconductor becomes one which is softand the coupling thereof between atoms is easily cut, and it is possibleto considerably relieve the strain involved in lattice mismatching. Thelow temperature buffer layer 42 may not be have to be In_(c) Ga_(1-c) Nbut the same result was obtained by In_(d) Al_(e) Ga_(1-d-e) N.

With respect to the material gas used for forming the foregoing lowtemperature buffer layer 42, in place of TMI, tertiary butyl phosphine(TBP) or tertiary butyl arsine (TBA), for example, is introduced, asemiconductor layer of a compound consisting of GaN_(w) P_(1-w) (0<w<1)or GaN_(v) As_(1-v) (0<v<1) is formed, and the same structure andmanufacturing method were used as in the case of In_(c) Ga_(1-c) N. Thegrowth temperature of the low temperature buffer layer 24 was 400° to600° C.

As a result, because a semiconductor of gallium nitride type compoundcontaining P or As is used for the low temperature buffer layer as asemiconductor layer which contacts a substrate such as sapphire, as inthe case of the example 1 in which such semiconductor contained In, itis possible to obtain a soft semiconductor layer with coupling thereofbetween atoms being easily cut, and to considerably relieve the straininvolved in the lattice mismatching.

In each of the foregoing example, examples of LED of double heterojunction were described, but it is the same with a laser diode of pnjunction various structures.

In accordance with the light emitting element of the semiconductor ofthe example 4, because a low temperature buffer layer consisting of asemiconductor containing at least In, P or As is provided as asemiconductor of gallium nitride type compound to be formed on thesurface of substrate, the semiconductor is soft and the strain isrelieved. As a result, occurrence of crystal defects and transition inthe low temperature buffer layer is restricted, progress of crystaldefects or transition toward the semiconductor which contributes toemission of light can also be restricted, thereby improving the lightemitting characteristic, improving reliability, and extending the lifefurther.

EXAMPLE 5

In the light emitting element of the semiconductor of this example 5, abuffer layer which is a semiconductor layer contacting a substrate of asemiconductor layer of gallium nitride type compound containing at leastan n-type layer and a p-type layer to be laminated to form a lightemitting portion on a substrate such as sapphire is composed by asemiconductor in which it is difficult for electric current to flow. Inother words, because the lattice constant of the semiconductor layer ofgallium nitride type compound and that of the sapphire substrate aredifferent in that the former is 4.758 Å and the latter is 3.189 Å,strain due to lattice mismatching occurs in the buffer larer on thesubstrate, and crystal defects or transition are likely to occur. Whenelectric current flows into the semiconductor where such crystal defectsor transition are occurring, heat is generated and crystal defects ortransition increase in the portion where cyrstal defects or transitionoccur. Because the crystal defects or transition occurred in this bufferlayer progress toward the semiconductor layer which forms the lightemitting portion, by preventing electric current form flowing to thebuffer layer on the substrate as much as possible, it is possible torestrict the occurrence of crystal defects or transition in the entiresemiconductor layer.

In order to make the buffer layer portion contacting the substrate as alayer where electric current is difficult to flow, it is possible toobtain such layer by making the layer as a high resistance layer or alayer of opposite conduction type by introducing a semiconductor layerof the upper part of the buffer layer and opposite conduction typedopant when growing a semiconductor layer in vapor phase. For example,because it is necessary to anneal and activate the p-type layer whenforming the light emitting element a semiconductor by laminating asemicondcutor of gallium type compound, the lower portion which is thesubstrate side is made into an n-type layer and the surface side is madeinto a p-type layer, and then are laminated. When growing asemicondcutor of gallium nitride type compound in vapor phase, N of thesemiconductor of gallium nitride type compound is easily evaporated, sothat the n-type layer is obtained without mixing a dopant. Therefore,when forming a buffer layer, by mixing the n-type clad layer to beformed on the buffer layer and the p-type dopant of opposite conductiontype, the layer to be originally formed into an n-type layer isneutralized by the p-type dopant and becomes a high resistance layer,and becomes a p-type layer by mixing more p-type dopant. Because then-side electrode is provided on the surface of the n-type layer of thebuffer layer or the clad layer, electric current does not flow to thehigh resistance layer of the substrate side or to the p-type layer. As aresult, electric current does not flow to the buffer layer of thesubstrate side in particular where crystal defects or transition arelikely to occur, so that it is possible to prevent an increase ofcrystal defects or transition

When a semiconductor of gallium nitride type compound containing atleast either In, P or As is used for the foregoing buffer layer, In isheavier than Ga and p or As is heavier than N, it is preferable that itis easy to relieve the strain because the coupling between atoms iseasily split, and it is further possible to prevent an increase ofcrystal defects or transition.

Next, the manufacturing method of the light emitting element of thesemiconductor of the example 5.

First, as shown in FIG. 10(a) of the example 4, on a substrate 1consisting of sapphire and the like is supplied by the MOCVD method thecarrier gas H₂ together TMG at the flow rate of 20 to 200 sccm, NH₃, atthe flow rate 5 to 20 slm, and Cp₂ Mg or DMZn at the flow rate of 10 to1000 sccm so as to grow in vapor phase at 400 to 700° C., and a lowtemperature buffer layer 42 consisting of GaN having the specificresistance of approximately 10 to 10¹⁸ ·cm is grown to approximatethickness of 0.01 to 0.2 μm.

Then, the temperature is raised to approximately 700° to 1200° C. tomake the foregoing low temperature buffer layer 42 into single crystal,and the same material gas as the foregoing gas is continuously supplied,the dopant gas is changed to SiH₄ and the high temperature buffer layer3 consisting of the n-type GaN is formed to approximate thickness of 2to 5 μm. When growing the high temperature buffer layer 3, it ispossible to obtain the n-type layer as described above even if the layeris grown without supplying the dopant gas, it is preferable to supplythe dopant gas in order to sufficiently increase the carrier gasdensity.

Afterwards, in the same manner as the example 4, semiconductors of then-type clad layer, active layer, p-type clad layer, and contact layerare laminated, annealed, and etched to be formed into the p-sideelectrode and the n-side electrode, and then made into chips.

In this example, the flow rate of the p-type dopant supplied duringgrowth of the low temperature buffer layer 42 is set to approximately 10to 100 sccm and the buffer layer is formed as a high resistance layerhaving the specific resistance of approximately 1000 to 10¹⁸ ·cm, but bysetting the flow rate of the p-type dopant to approximately 500 to 1000sccm and forming the layer as the p-type layer, the high temperaturebuffer layer 3 thereon is the n-type and the n-side electride isprovided on the n-type layer, therefore, little electric current flowsto the p-type layer on the insulated substrate, and it is possible toform a layer where electric current is difficult to flow.

In addition, in this example, the entire high temperature buffer layer 3on the low temperature buffer layer 42 is formed by the example of then-type layer, but it is also possible to make the lower layer side ofthe high temperature buffer layer 3 as the high resistance layer or thep-type layer. In this case, by changing only the dopant gas to besupplied while growing a semiconductor layer in vapor phase, it ispossible to change the conduction type dopant. Further, the entire ofthe high temperature buffer layer 3 may be made a high resistance layeror a p-type layer and the n-type clad layer may be made to a thicknessto an extent where the series resistance does not pose a problem.

Further, in the light emitting element of a semiconductor which uses asemiconductor of gallium nitride type compound, as stated above,normally the n-type layer is formed n the lower layer side close to thesubstrate and p-type layer is formed on the surface side, but even ifthe n-type layer and the p-type layer are formed in the opposite way, itis possible to form a layer where the same electric current is difficultto flow by simply reversing the n-type layer and the p-type layer.

In addition, in this example and LED of double hetero juction isdescribed, the present invention can be applied to the light emittingelement of semiconductor such as laser diode having LED of pn junctionor various structures.

In accordance with the light emitting element of the semiconductor ofthe example 5, because the semiconductor layer of gallium nitride typecompound (buffer layer) contacting the substrate where crystal defectsor transition are likely to occur due to the lattice mismatching is madea layer where electric current is difficult to flow, it is possible toprevent an increase of crystal defects or transition caused by electriccurrent. As a result, it is possible to restrict the occurrnce ofcrystal defects or transition in the semiconductor layer composing thelight emitting portion, and it is possible to obtain the light emittingelement of semiconductor having excellent light emitting characteristic.

Further, crystal defects or transition do not increase during operationdue to the influence of electric current, while the reliability isimproved and the life is extended as well.

In each example of the foregoing examples 1 through 5, examples ofsemiconductor layers of special compositions are described as thesemiconductor layer of gallium nitride type compound, but not limitingto the material of the foregoing composition, the semiconductor layer isgenerally consisted of Al_(p) Ga_(1-p-q) N (0≦p<1, 0<q≦1, 0<p+q≦1), theratio of each composition may be selected so that the bond gap energy ofthe active layer, for example, is smaller than the band gap energy ofthe clad layer, or the composition ratios p and q may be changed so asto satisfy the band gap energy or the refractive index of eachsemiconductor layer according to the desired light emitting element ofsemiconductor. The present invention can be applied in the same manneras to the material in which part or whole of N of the foregoing Al_(p)Ga_(1-p-q) N is replaced by As and/or P.

What is claimed is:
 1. A semiconductor laser of double hetero junctionstructure comprising:an active layer; an n-type layer and a p-typelayer, sandwiching said active layer, a band gap energy of said n-typelayer and said p-type layer being larger than that of said active layerand a refractive index of said n-type layer and said p-type layer beingsmaller than that of said active layer; wherein said n-type and p-typelayers comprises at least two layers, respectively; wherein in saidactive layer side of said n-type and p-type layers is provided a lowrefractive index layer formed of a material of small refractive index,in other portion of the electric current path of said n-type layer andp-type layer is provided a low resistance layer having smaller electricresistance than that of said low refractice index layer.
 2. Thesemiconductor laser according to claim 1, wherein the thickness of saidlow refractive index layer is 10 to 50% with respect to the thickness ofsaid n-type layer or p-type layer.
 3. The semiconductor laser accordingto claim 1, wherein the thickness of said low refractive index layer is0.05 to 0.3 μm.
 4. The semiconductor laser according to the claim 1,wherein said active layer consists of Al_(m) Ga_(n) In_(1-m-n) N (0≦m<1,0<n<1, 0<m+n<1), said low refractive index layer comprising Al_(r)Ga_(s) In_(1-r-s) N (0<r<1, 0<s<1, m+n<r+s≦1, m<r), and said lowresistance layer comprising Al_(t) Ga_(u) In_(1-t-u) N (0≦t<1, 0<u≦1,0<t+u≦1, m≦t<r, m+n<t+u≦r+s).
 5. The semiconductor laser according toclaim 4, wherein m=0 in the material composition of said active layer,r+s=1 in the material composition of said low refractive index layer,and t=o and u=1 in the material composition of said low resistancelayer.
 6. A semiconductor light emitting device comprising:a substrate;and GaN-type compound semiconductor layers stacked on the substrate, theGaN-type layers including at least one active layer, at least one n-typelayer, and at least one p-type layer; wherein a band gap energy of theone n-type layer is smaller than a band gap energy of the one p-typelayer.
 7. The semiconductor light emitting device of claim 6, wherein aband gap energy of the active layer is smaller than a band gap energy ofeach of the one n-type layer and the one p-type layer.
 8. Thesemiconductor light emitting device of claim 7, wherein a differencebetween the band gap energies of the one n-type layer and the one activelayer is not more than 1/2 of a difference between the band gap energiesof the one p-type layer and the active layer.
 9. The semiconductor lightemitting device of claim 8, wherein the difference between the band gapenergies of the one n-type layer and the active layer is in a range of1/3 to 1/2 of the difference between the band gap energies of the onep-type layer and the active layer.
 10. A semiconductor light emittingdevice of GaN-type compound comprising:an active portion located betweena p-type portion and an n-type portion; wherein a maximum band gapenergy of the n-type portion is smaller than a maximum band gap energyof the p-type portion.
 11. The semiconductor light emitting device ofclaim 10, wherein one of the p-type portion includes Al_(x) Ga_(1-x) N(0<x<1).
 12. The semiconductor light emitting device of claim 10,wherein the device is one of a light emitting diode and a laser diode.13. The semiconductor light emitting device of claim 10, wherein aminimum band gap energy of the active portion is smaller than each ofthe maximum band gap energy of the p-type portion and the maximum bandgap energy of the n-type portion.
 14. The semiconductor light emittingdevice of claim 13, wherein a difference between the maximum band gapenergy of the n-type portion and a minimum band gap energy of the activeportion is not more than 1/2 of a difference between the maximum bandgap energy of the p-type portion and the minimum band gap energy of theactive portion.
 15. The semiconductor light emitting device of claim 14,wherein the difference between the maximum band gap energy of the n-typeportion and the minimum band gap energy of the active portion is in arange of 1/3 to 1/2 of a difference between the maximum band gap energyof the p-type portion and the minimum band gap energy of the activeportion.
 16. The semiconductor light emitting device of claim 10,further including:a buffer portion located between a substrate and oneof the n-type and p-type portions.
 17. A GaN-type semiconductor lightemitting device comprising:an active portion located between a p-typeportion and an n-type portion, the active portion emitting light byconfining an electron and a positive hole, the electron having adifficulty of escaping from the active portion to the p-type portionbased on an effective mass of the electron, the positive hole having adifficulty of escaping from the active portion to the n-type portionbased on an effective mass of the positive hole, wherein acharacteristic of one of the p-type portion and the n-type portion isadjusted such that the difficulty of the electron of escaping from theactive portion is similar to the difficulty of the positive holeescaping from the active portion.
 18. The semiconductor light emittingdevice of claim 17, wherein the characteristic is a band gap energy. 19.A semiconductor light emitting diode comprising:an active portionlocated between a p-type portion and an n-type portion, the activeportion having a minimum band gap energy smaller than each of a maximumband gap energy of the p-type portion and the n-type portion, whereinthe maximum band gap energy of the n-type portion is smaller than themaximum band gap energy of the p-type portion.
 20. The semiconductorlight emitting diode of claim 19, wherein one of the p-type portionincludes Al_(x) Ga_(1-x) N (0<x<1).
 21. The semiconductor light emittingdiode of claim 20, wherein a difference between the maximum band gapenergy of the n-side portion and the minimum band gap energy of theactive portion is not more than 1/2 of a difference between the maximumband gap energy of the p-type portion and the minimum band gap energy ofthe active portion.
 22. The semiconductor light emitting diode of claim21, wherein the difference between the maximum band gap energy of then-side portion and the minimum band gap energy of the active portion isin a range of 1/3 to 1/2 of a difference between the maximum band gapenergy of the p-type portion and the minimum band gap energy of theactive portion.
 23. The semiconductor light emitting diode of claim 10,further including:a buffer portion located between a substrate and oneof the n-type and p-type portions, the buffer portion including GaN.